Arrhythmia detection and defibrillation system and method

ABSTRACT

The invention relates to an arrhythmia detection system and method for defibrillating the heart of a patient experiencing abnormal cardiac rhythm, wherein the abnormal cardiac rhythm (comprising one of fibrillation, high rate tachycardia, and low rate tachycardia) is first detected, the heart rate is sensed so as to distinguish between fibrillation and high rate tachycardia, on the one hand, and low rate tachycardia, on the other hand, and automatic defibrillation of the heart of the patient is implemented when one of fibrillation and high rate tachycardia is determined. In one embodiment, base and apical electrodes are connected to a probability density function (PDF) circuit and a rate circuit. When both abnormal cardiac rhythm and excessively high heart rate are detected, defibrillation of the heart of the patient is implemented. In a second embodiment, a sensing button is also connected to the heart, and a switch is interposed between the electrodes and sensing button, on the one hand, and the interface, on the other hand. During monitoring of cardiac rhythm by the PDF circuit, the switch automatically connects the electrodes to the PDF circuit, while, during sensing of the heart rate, the switch connects the sensing button to the heart rate circuit. Upon detection of the need for defibrillation, the switch automatically connects the defibrillation pulse generator to the electrodes. Further features include a timed reset capability.

The present application is a continuation of prior U.S. patentapplication Ser. No. 175,670, filed Aug. 5, 1980, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an arrhythmia detection system andmethod, and more particularly to an improved system and method fordefibrillating the heart of a patient when the patient experienceslife-threatening fibrillation.

2. Description of the Prior Art

In recent years, substantial progress has been made in the developmentof defibrillation techniques for providing an effective medical responseto various heart disorders or arrhythmias particularly ventricularfibrillation which is characterized by an irregular ECG waveform.Earlier efforts resulted in the development of an electronic standbydefibrillator which, in response to detection of abnormal cardiacrhythm, discharged sufficient energy, via electrodes connected to theheart, to depolarize the heart and restore it to normal cardiac rhythm.Examples of such electronic standby defibrillators are disclosed incommonly assigned U.S. Pat. No. 3,614,954 (subsequently, U.S. Pat No.Re. 27,652) and U.S. Pat. No. 3,614,955 (subsequently, U.S. Pat. No. Re.27,757).

Past efforts in the field have also resulted in the development ofimplantable electrodes for use in accomplishing ventriculardefibrillation (as well as other remedial techniques). In accordancewith such techniques, as disclosed (for example) in U.S. Pat. No.4,030,509 of Heilman et al, an apex electrode is applied to the externalintrapericardial or extrapericardial surface of the heart, and actsagainst a base electrode which can be either similarly conformal or inthe form of an intravascular catheter. Such electrode arrangements ofthe prior art, as disclosed in the aforementioned patent of Heilman etal, can employ independent pacing tips associated with either a baseelectrode or an apex electrode, or both.

Recent efforts also have resulted in the development of techniques formonitoring heart activity (for the purpose of determining whendefibrillation or cardioversion is necessary), which techniques employ aprobability density function for determining when ventricularfibrillation is present. Such a technique, employing the probabilitydensity function, is disclosed in U.S. Pat. Nos. 4,184,493 and4,202,340, both of Langer et al.

In accordance with this latter technique of the prior art, when theprobability density function is satisfied, fibrillation of the heart isindicated. However, recent experience has shown that, with one or moreparticular abnormal ECG patterns, the prior art probability densityfunction detector, if not optimally adjusted, can be "triggered" notonly by actual ventricular fibrillation, but also by some forms of highrate ventricular tachycardia, and low rate ventricular tachycardia aswell, particularly in the presence of ventricular conductionabnormalities. Unlike ventricular fibrillation, such high rate and lowrate tachycardias are characterized by regular R-waves occurring atgenerally stable rates. The possibility of such triggering in thepresence of high rate tachycardia is acceptable because high ratetachycardia can be fatal if present at such a rate that sufficient bloodpumping no longer is accomplished. However, triggering in the presenceof non-life threatening, low rate tachycardia could be considered aproblem. Therefore, it has been determined that there is a need for asystem and method for distinguishing between ventricular fibrillationand high rate tachycardia, on the one hand, and low rate tachycardia, onthe other hand.

It is worth noting that prior art implementation of the probabilitydensity function technique was, for a time, limited to "triggering" onlyin the presence of ventricular fibrillation. This was accomplished byadjusting the decision boundaries of the probability density functiondetector in a conservative way so as to "trigger" only upon occurrenceof life-threatening ventricular fibrillation. However, it was soonrealized that there existed situations in which it was desirable to takeremedial action upon the detection of high rate tachycardia, asindicated by occurrence of a heart rate above a lower threshold level(for example, above 200 beats per minute). This was initiallyaccomplished merely by adjusting the probability density functioncriteria so as to be "triggered" at the lower threshold level.

However, it was soon discovered that a problem existed in a detectorwith relaxed decision criteria, in that extraordinary types of ECGsignals were capable of "triggering" the modified probability densityfunction detector, even though neither ventricular fibrillation nor highrate tachycardia was present. Therefore, it has been determined thatthere is a need for an arrhythmia detection system and method which notonly performs a probability density function analysis, but which alsoincludes some technique for distinguishing between fibrillation and highrate tachycardia, on the one hand, and low rate tachycardia, on theother hand. Thus, the inventive system and method herein disclosedamounts to a "backup" technique by means of which high rate tachycardiais treated by issuance of a defibrillating shock to the patient, whilelow rate tachycardia is not so treated.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an arrhythmiadetection system and method, and more particularly an improved systemand method for defibrillating a heart which is undergoing abnormalcardiac rhythm, the improved system and method employing a technique fordistinguishing between ventricular fibrillation and high ratetachycardia, on the one hand, and low rate tachycardia, on the otherhand. More specifically, the system and method of the present invention,besides utilizing the probability density function technique todetermine the presence of abnormal cardia rhythm, also employs heartrate sensing for the purpose of distinguishing between ventricularfibrillation and high rate tachycardia, on the one hand, the latterbeing indicated by a heart rate above a predetermined threshold, and lowrate tachycardia, on the other hand, the latter being indicated by aheart rate falling below the predetermined threshold.

The present invention is implemented by a first preferred embodiment ofa system, wherein a superior vena cava (or base) electrode and an apical(or patch) electrode are associated with the heart, and are employed, asin conventional in the art, not only to derive an electrocardiograph(ECG) signal, but also to apply a defibrillating shock to the heart. Itshould be noted that the ECG amplifier in the first embodimentessentially provides the derivative of the heart signal as taught inU.S. Pat. No. 4,184,493. However, in contrast to the prior art, thedifferentiated ECG signal is, in this first embodiment of the invention,applied to a probability density function circuit, and to a low passfilter and a heart rate circuit, by means of which the probabilitydensity function and heart rate, respectively, are obtained. Inaccordance with this first embodiment, satisfaction of the probabilitydensity criteria (that is, determination of whether the time-averagedderivative of the ECG remains off the base line for extended periods oftime) while the heart rate is above a predetermined threshold results inactuation of a conventional defibrillating pulse generator to issue adefibrillating shock to the heart. Thus, the defibrillating shock willbe issued to the heart only upon the occurrence of fibrillation or highrate tachycardia, as contrasted with non-life threatening low ratetachycardia.

In accordance with a second embodiment of the present invention, asensing button (preferably, associated with the apical or patchelectrode) is connected to the heart for use in deriving the heart rate.Thus, in this embodiment, the base and apical electrodes are utilizedinitially to derive the ECG signal by means of which the probabilitydensity function is examined. If the probability density functionindicates abnormal cardiac rhythm, a switching operation takes place,whereby the sensing button is utilized to derive an ECG signal which isfurther utilized to determine the heart rate. Since a very small areaelectrode will result in signals in which cardiac depolarizations canstill be identified, even during ventricular fibrillation, aconventional R-wave detector can be used so as to provide an R-wave forheart rate sensing. Then, if the heart rate is above the predeterminedthreshold, a defibrillating shock is issued. Once a shock is issued, afurther switching operation is executed so that the base and apicalelectrodes may be utilized in further examining the probability densityfunction.

In accordance with a further feature of the present invention, thissecond embodiment is provided with a timed reset capability, whereby,once the probability density function indicates abnormal cardiac rhythm,if a heart rate above the predetermined threshold is not indicatedwithin a predetermined time, a return switch operation is automaticallyexecuted so as to permit renewed monitoring of the base and apicalelectrodes and examination of the resulting ECG signal vis-a-vis theprobability density function.

Therefore, it is an object of the present invention to provide anarrhythmia detection system and method, and more particularly animproved system and method for defibrillating a heart experiencingabnormal cardiac rhythm.

It is a further object of the present invention to provide a system andmethod which is capable of distinguishing between ventricularfibrillation and high rate tachycardia, on the one hand, and low ratetachycardia, on the other hand.

It is a further object of the present invention to provide a system andmethod which employs a probability density function technique todetermine the existence of abnormal cardiac rhythm, and which furtheremploys a heart rate sensing technique for the purpose of distinguishingbetween ventricular fibrillation and high rate tachycardia, on the onehand, and low rate tachycardia, on the other hand.

It is a further object of the present invention to provide a system andmethod, wherein base and apical electrodes are employed both formonitoring the ECG signal vis-a-vis the probability density function,and for determining whether or not the heart rate is above or below apredetermined threshold.

It is an additional object of the present invention to provide a systemand method which employs base and apical electrodes for monitoring theECG signal in conjunction with examining the ECG signal vis-a-vis theprobability density function, and which employs a sensing button toderive heart rate information for distinguishing between ventricularfibrillation and high rate tachycardia, on the one hand, and low ratetachycardia, on the other hand.

It is an additional object of the present invention to provide a systemand method, wherein the probability density function is first examinedto determine whether or not abnormal cardiac rhythm exists, and wherein,if such abnormal cardiac rhythm does exist, the heart rate of thepatient is then examined to distinguish between ventricular fibrillationand high rate tachycardia, on the one hand, in which case adefibrillation pulse is issued, and low rate tachycardia, on the otherhand, in which case a defibrillation pulse is not issued.

It is an additional object of the present invention to provide a systemand method with a timed reset capability, wherein, once the probabilitydensity function indicates abnormal cardiac rhythm, if a heart rateexceeding a predetermined threshold is not detected within apredetermined amount of time, the system and method return to monitoringof the ECG signal vis-a-vis the probability density function, and nodefibrillation pulse is issued.

The above and other objects that will hereinafter appear, and the natureof the invention, will be more clearly understood by reference to thefollowing description, the appended claims, and the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a first embodiment of the arrhythmiadetection system of the present invention.

FIG. 2 is a detailed diagram of the heart rate circuit employed fordetecting heart rate in the embodiment of FIG. 1.

FIG. 3A and 3B are a series of waveform diagrams utilized in describingthe operation of the heart rate circuit of FIG. 2.

FIG. 4 is a block diagram of a second embodiment of the system of thepresent invention.

FIG. 5 is a detailed diagram of the heart rate circuit employed fordetecting heart rate in the embodiment of FIG. 2.

FIG. 6 is a series of waveform diagrams utilized in describing theoperation of the heart rate circuit of FIG. 5.

DETAILED DESCRIPTION

The arrhythmia detection system and method of the present invention willnow be described in more detail with reference to FIG. 1, which is ablock diagram of a first embodiment of the system.

Referring to FIG. 1, the system of the present invention (generallyindicated by reference numeral 10) is connected to a superior vena cava(or base) electrode 12 and an apical (or patch) electrode 14, the latterbeing disposed in contact with the heart of the patient as is known inthe prior art (see, for example, U.S. Pat. No. 4,030,509 of Heilman etal mentioned above). In the system 10, electrodes 12 and 14 areconnected, via an interface 16, to an ECG amplifier 18 which hasinherent filtering so as to provide an approximation of thedifferentiated ECG.

The ECG amplifier 18 is connected both to low pass filter circuit 19(which itself is connected to heart rate circuit 21) and to probabilitydensity function (PDF) circuit 20. Heart rate circuit 21 is connectedvia its inhibit output line (INHIBIT) to the PDF circuit 20, by means ofwhich the output line heart rate circuit 21 is able to inhibit outputfrom the PDF circuit 20. The output of the PDF circuit 20 is connectedto defibrillation pulse generator 26, the latter being connected via theinterface 16 to the electrodes 12 and 14.

In operation, electrodes 12 and 14 are employed, via the interface 16 (aconventional interface, or isolation circuit), for two purposes: (1)monitoring of heart activity via ECG amplifier 18, which develops adifferentiated ECG signal output provided to PDF circuit 20 and low passfilter 21, respectively; and (2) application of a defibrillating shockfrom defibrillation pulse generator 26, via the interface 16, to theheart. More specifically, PDF circuit 20 monitors the probabilitydensity function of the differentiated ECG output signal of ECGamplifier 18, and, in accordance with conventional techniques (asdisclosed, for example, in U.S. Pat. Nos. 4,184,493 and 4,202,340 ofLanger et al), determines when abnormal cardiac rhythm of the heartexists. At the same time, the low pass filtered ECG signal, provided asthe output of low pass filter 19, is employed by rate circuit 21 todetermine when the heart rate exceeds a predetermined threshold, atwhich time rate circuit 21 removes its inhibiting influence on PDFcircuit 20.

Thus, upon determination of abnormal cardiac rhythm by PDF circuit 20,and of a heart rate above the predetermined threshold by rate circuit22, the PDF circuit 20 is allowed to enable defibrillation pulsegenerator 26, causing the latter to apply a defibrillating shock, viainterface 16, to the heart.

FIG. 2 is a detailed diagram of the heart rate circuit employed fordetecting heart rate in the embodiment of FIG. 1, while FIGS. 3A and 3Bare a series of waveform diagrams utilized in describing the operationof the heart rate circuit of FIG. 2.

As seen in FIG. 2, the heart rate circuit 21 comprises operationalamplifier OP1 (which is used as a comparator), transistors Q1 throughQ4, resistors R3 through R14, capacitors C2 and C3, and diodes D1 andD2.

In operation, the heart rate circuit 21 of FIG. 2 functions in thefollowing manner, with reference to the waveforms shown in FIG. 3. Aspreviously mentioned, the input to ECG amplifier 18 (FIG. 1) comprisesan undifferentiated ECG signal, as provided by the electrodes 12 and 14.The undifferentiated ECG signal is represented by the waveform 100 ofFIG. 3A, and is illustrated as a signal having regular R-waves and agenerally stable (or uniform) rate.

As also previously mentioned, the ECG amplifier 18 amplifies and filters(differentiates) the ECG signal, and the amplified and differentiatedoutput of the ECG amplifier 18 is shown as waveform 102 in FIG. 3A.Finally, prior to being provided as an input to the heart rate circuit21, the amplified and differentiated ECG signal from amplifier 18 isfiltered, in a manner to be discussed in more detail below, by low passfilter 19 (made up of resistor Rl and capacitor Cl).

Referring to FIG. 2, the amplified, differentiated and filtered ECGsignal is provided to the negative input of operational amplifier OP1,the positive input of which receives a reference input REF via resistorR3. Operational amplifier OP1 is used as a comparator, and switchesbetween low and high outputs in accordance with the relationship betweenthe ECG input and the reference REF. More specifically, it should benoted that zero crossings in a derivative waveform (such as the outputof amplifier 18 of FIG. 1) correspond to peaks in the original signal(the original ECG signal). Accordingly, low pass filter 19 of FIG. 1filters the differentiated ECG input provided thereto in such a way thatthe output of operational amplifier OP1 (FIG. 2), used as a comparator,switches at the zero crossings in the derivative waveform correspondingto major peaks in the ECG input signal. The output of comparator OP1appears as waveform 104 in FIG. 3A, and illustrates the switching actionjust described.

Further referring to FIG. 2, transistor Q1 has its emitter connected toone of the offset adjustment terminals of operational amplifier OP1 soas to add hysteresis to the switching threshold of the amplifier OP1.This hysteresis, in combination with the characteristics of the low passfilter 19 of FIG. 1, has the effect of reducing the sensitivity of theheart rate circuit 21 to smaller peaks in the ECG input signal. As willbe now described, the remainder of heart rate circuit 21 of FIG. 2 actsas a precision timer which is responsive to the ECG peaks, as detectedby and indicated by switching of the operational amplifier OP1.

Specifically, a programmable uni-junction transistor Q2 is connected inseries with resistor R5, the latter series combination being connectedbetween the output of amplifier OP1 and the collector of transistor Q1.In addition, the gate lead of transistor Q2 is connected, via resistorR4 to the output of amplifier OP1. In short, programmable uni-junctiontransistor Q2 is connected in such a way as to provide a narrow pulse tothe base of a further transistor Q3 having its base connected viaresistors R7 and R8 to the uni-junction transistor Q2, as shown. Thenarrow pulse thus provided to the base of transistor Q3 corresponds tothe rising edge of the output of amplifier OP1 (waveform 104 of FIG.3A), and this narrow pulse is indicated by waveform 106 of FIG. 3A. Theoperation of programmable uni-junction transistor Q2, which thusprovides the pulse output shown in waveform 106, will be evident tothose of skill in the art with respect to the utilization of suchdevices.

The narrow pulse shown in waveform 106 of FIG. 3A is also shown in FIG.3B. This narrow pulse is applied to the base of transistor Q3, and turnson transistor Q3 at a frequency determined by the frequency ofoccurrence of the rising edges of the output of amplifier OP1, that is,in accordance with a frequency related to heart rate. Thus, if the heartrate is sufficiently high, capacitor C2 will experience a voltagebuild-up as shown in waveform 108 of FIG. 3B. That is to say, capacitorC2 will experience a build-up of voltage under the influence of powersupply V_(s) (provided via resistors R9 and RlO), and will thendischarge through transistor Q3 when that transistor is turned on byreceipt of a narrow pulse (waveform 106 of FIG. 3B) at the base oftransistor Q3. Accordingly, since the voltage of C2, for high heartrate, will not reach the threshold voltage of programmable uni-junctiontransistor Q4, transistor Q4 will not conduct, and the gate voltage oftransistor Q4 (that is, the voltage at the junction between resistorsRll and R12, diode D2 and transistor Q4) will remain high (see waveform110 of FIG. 3B). Thus, the output INHIBIT of rate circuit 21 will remainhigh, and will not inhibit the PDF circuit 20 of FIG. 1.

Conversely, if the heart rate is low, conduction of transistor Q3 willbe relatively less frequent, and capacitor C2 will charge to the pointwhere transistor Q4 will "fire", thus discharging capacitor C2therethrough. This charging and discharging of capacitor C2, under theseconditions, is indicated by waveform 112 of FIG. 3A. When transistor Q4"fires" in this manner, its gate lead is pulled low, and remains lowuntil receipt of the next narrow pulse applied to the base of transistorQ3. Specifically, the next narrow pulse received at the base oftransistor Q3 provides (as indicated in waveform 112 of FIG. 3A) aslight negative voltage on capacitor C2, and this slight negativevoltage turns off transistor Q4, returning it to its non-conductivestate. Accordingly, the voltage at the point between resistors Rll andR12, diode D2 and transistor Q4 returns to a positive polarity, asindicated by waveform 114 of FIG. 3A.

Thus, firing of transistor Q4 results in the occurrence of anegative-going pulse (waveform 114 of FIG. 3A) at the aforementionedjunction between resistors R11 and R12, diode D2 and transistor Q4. Suchnegative-going pulses are utilized to inhibit operation of the PDFcircuit 20 of FIG. 1 (via the control line INHIBIT). Specifically, thesenegative-going pulses are utilized to remove charge from the integratingcapacitor in the PDF circuit 20, as taught in U.S. Pat. No. 4,184,493 ofLanger et al.

To summarize, the operation of PDF circuit 20 (FIG. 1) is inhibited byrate circuit 21 at low heart rate, but no such inhibiting function takesplace at high heart rate. Thus, at high heart rate, the PDF circuit 20proceeds with its normal detection operation, and enables defibrillationpulse generator 26 in accordance therewith.

Further referring to FIG. 2, it is to be noted that diode D2 is providedfor the purpose of temperature and voltage stabilization of the timeinterval of transistor Q4.

Referring back to FIG. 1, as mentioned earlier, interface 16 is aconventional interface. More specifically, interface 16 protects the ECGamplifier 18 from the defibrillation pulses issued by generator 26,while at the same time permitting the monitoring of heart activity byECG amplifier 18. Interface 16 is, for example, disclosed in more detailin copending application U.S. Ser. No. 215,520 of Langer, entitled"Method and Apparatus for Combining Pacing and Cardioverting Functionsin a Single Implanted Device." Moreover, the PDF circuit 20 is aconventional circuit for performing the probability density function,and is, for example, disclosed in more detail in the aforementioned U.S.Pat. Nos. 4,184,493 and 4,202,340 of Langer et al.

FIG. 4 is a block diagram of a second embodiment of the system of thepresent invention. Elements common to both FIGS. 1 and 4 have beenidentified by identical reference numerals.

Referring to FIG. 4, the system 30 is shown connected to base and apicalelectrodes 12 and 14, and to a sensing button 32 (associated with apicalelectrode 14). More specifically, the electrodes 12 and 14 and sensingbutton 32 are connected, via a switch 34 and interface 16, both to theECG amplifier 18 and the defibrillation pulse generator 26. ECGamplifier 18 is connected, as was the case in FIG. 1, to the PDF circuit20, but is also connected to R-wave detector 22 which is of conventionaldesign and provides a pulse with each R-wave. R-wave detector 22 issubsequently connected to rate circuit 23, which is shown in detail inFIG. 5 (to be discussed below). The output of PDF circuit 20 isconnected, via flip-flop 36, to one input of the AND gate 24, the otherinput of which is connected to the output of rate circuit 23. The outputof flip-flop 36 is connected to rate circuit 23, the input of a timedreset circuit 38 (the output of which is connected to the "reset" inputof flip-flop 36), and to switch 34. Moreover, the output of AND gate 24is connected not only to defibrillation pulse generator 26, but also tothe "reset" input of flip-flop 36 and to switch 34.

In operation, switch 34 is initially in the position indicated byreference numeral 40. Therefore, in this mode (subsequently referred toas the "patch" mode), the base and apical electrodes are utilized by ECGamplifier 18 which monitors heart activity via interface 16, switch 34,and aforementioned electrodes 12 and 14. The resulting ECG signal outputfrom amplifier 18 is provided to PDF circuit 20 (rate circuit 23 isinitially in the "off" state). Detection of abnormal cardiac rhythm byPDF circuit 20 results in generation of an output, which is applied tothe "set" input of flip-flop 36. When flip-flop 36 is set, existence ofabnormal cardiac rhythm is "memorized", and a Q output is generated.

The Q output of flip-flop 36 is applied as an "enabling input" to ANDgate 24. It is also applied as a START command to both rate circuit 23and timed reset circuit 38. Moreover, the Q output of flip-flop 36 isprovided as signal SENSE to the switch 34, resulting in actuation ofswitch 34 to the position indicated by reference numeral 42. Thisestablishes the "sense" mode of operation, during which heart rate ismonitored by rate circuit 23. More specifically, actuation of switch 34to the position indicated by reference numeral 42 connects the interface16 to the sensing button 32, so that heart rate can be monitored by ratecircuit 23 via R-wave detector 22, switch 34, interface 16, and ECGamplifier 18. Since the ECG amplifier 18 is connected to a very smallsurface area electrode, sharp depolarizations will still be delivered toR-wave detector 22, resulting in a proper signal indication of heartrate. It will be recalled that rate circuit 23 was started by the Qoutput of flip-flop 36, the latter being issued as a result of detectionof abnormal cardiac rhythm (satisfaction of the probability densityfunction criteria) by PDF circuit 20.

If and when rate circuit 23 detects a heart rate which exceeds apredetermined threshold, it issues an output to AND gate 24 which, asenabled by the Q output of flip-flop 36, provides this output as anenabling input to defibrillation pulse generator 26. Moreover, AND gate24 provides this output to the "reset" input of flip-flop 36 (thus,resetting flip-flop 36), and as an input signal PATCH to switch 34,actuating switch 34 to the position indicated by reference numeral 40,thus reestablishing the "patch" mode of operation of the system 30.Finally, defibrillation pulse generator 26, as enabled by AND gate 24,issues a defibrillation pulse, via interface 16 and switch 34 (inposition 40), to the base and apical electrodes 12 and 14, respectively,so as to defibrillate the heart of the patient.

As previously mentioned, the Q output of flip-flop 36 starts the timedreset circuit 38 upon detection of abnormal cardiac rhythm by the PDFcircuit 20. If, after a predetermined period of time, rate circuit 23has not detected a heart rate above the predetermined threshold, timedreset circuit 38 automatically issues a "reset" input to flip-flop 36,and provides a further input PATCH to the switch 34, so as to actuateswitch 34 to the position indicated by reference numeral 40, thusreestablishing the "patch" mode of operation. Accordingly, the system 30is provided with the beneficial feature whereby if, within apredetermined time after detection of abnormal cardiac rhythm by PDFcircuit 20, heart rate is not detected as exceeding the predeterminedthreshold, the system 30 is returned to the "patch" mode of operation soas to permit further monitoring of the ECG signal by the PDF circuit 20.That is to say, the timed reset circuit 38 removes the enabling inputfrom AND gate 24, turns off the rate circuit 23, and returns the switch34 to the "patch" position (indicated by reference numeral 40). Then thePDF circuit 20 monitors the base and apical electrodes 12 and 14,respectively, via switch 34, interface 16 and ECG amplifier 18, to onceagain detect existence of any abnormal cardiac rhythm.

FIG. 5 is a detailed diagram of the heart rate circuit 23 of FIGS. 4,while FIG. 6 is a series of waveform diagrams describing the operationof the heart rate circuit 23 of FIG. 4. As seen in FIG. 5, rate circuit23 comprises input resistor 50, NPN transistor 52, current source 54,capacitor 56, differential amplifier or comparison circuit 58, peakdetector 60, shift register 62 and AND gate 64.

In operation, ECG signals (generally indicated by reference numeral 70of FIG. 6 and shown, for each of the two discrete segments, as havingregular R-waves and a generally stable rate) are provided to R-wavedetector 22 (FIG. 4), the latter generating a pulse train (generallyindicated by reference numeral 75 of FIG. 6) corresponding thereto.Specifically, this pulse train output of the R-wave detector 22 isprovided, via input resistor 50 (FIG. 5), to the base of NPN transistor52. Transistor 52 is turned on as a result of receipt of each individualpulse in pulse train 75, and is thus turned on in correspondence todetection of individual R-waves 72, 74. During those periods of timebetween individual R-waves 72 (or 74) in FIG. 6, transistor 52 isnon-conductive, and current source 54 builds up a voltage on capacitor56. This build-up of voltage on capacitor 56 is generally indicated bywaveform 76 of FIG. 6.

However, upon occurrence of an R-wave 72 or 74, NPN transistor 52 (FIG.5) becomes conductive, and capacitor 56 discharges therethrough (seeindividual waveforms 78 and 80 of FIG. 6). Thus, it can be seen that,for a normal heart rate (as indicated by waveforms 72 of FIG. 6),capacitor 56 discharges at a relatively low frequency of discharge;thus, current source 54 is able to build the voltage across capacitor 56to a relatively high level, exceeding a predetermined reference REF(indicated by reference numeral 86 in FIG. 6). Conversely, occurrence ofR-waves at an abnormally high rate (as indicated by waveforms 74 of FIG.6) results in discharge of capacitor 56 at a more frequent rate (asindicated by waveforms 80 of FIG. 6), and the reference REF is notexceeded.

Further referring to FIG. 5, it is seen that differential amplifier 58is provided, at its negative input, with a voltage corresponding to thevoltage built up on capacitor 56, and, at its positive input, with avoltage REF corresponding to the predetermined reference level 86 ofFIG. 6. Thus, referring to FIGS. 5 and 6, whenever the voltage oncapacitor 56 exceeds the predetermined reference 86 (as in waveforms78), differential amplifier 58 issues an output X equal to 0, asindicated by inverted square waves 84 of FIG. 6, to the shift register62. Conversely, during those periods of time when the voltage acrosscapacitor 56 does not exceed the reference 86 (as in waveforms 80 ofFIG. 6), differential amplifier 58 issues an output X equal to 1 to theshift register 62.

Rate circuit 23 of FIG. 5 is further provided with a peak detector 60,which is a conventional circuit for detecting the existence of peaks inthe R-waves 70 of FIG. 6. Upon detection of each peak, detector 60issues an input SHIFT to shift register 62. As a result, the output X,at that time, of differential amplifier 58 is shifted into an end stageof shift register 62, the contents of register 62 being accordinglyshifted by one place to the right.

Furthermore, the output of shift register 62 (corresponding to thecontents of each bit or stage thereof) is provided to AND gate 64. Onlydetection of all 1's in shift register 62 will result in an output fromAND gate 64. This output of AND gate 64 is provided to one input of ANDgate 24 (FIG. 4), the other input of which receives the Q output offlip-flop 36, indicating satisfaction of the probability densityfunction criteria, as determined by PDF circuit 20. As a result, withboth the probability density function criteria having been satisfied andan excessive heart rate having been detected, AND gate 24 enablesdefibrillation pulse generator 26, so as to issue a defibrillation pulseto the heart of the patient. Conversely, so long as any of the bits ofshift register 62 are 0 in value, AND gate 64 does not issue an output,indicating that a consistently high heart rate has not been detected byrate circuit 23. Thus, shift register 62 provides a means of rememberingthe rates of previous beats. (It should be evident that the greater thenumber of bits in the shift register, the higher is the number ofR-waves which must exceed a given rate before a high rate is indicated.)Thus, defibrillation does not take place, even if the probabilitydensity function is satisfied.

While preferred forms and arrangements have been shown in illustratingthe invention, it is to be clearly understood that various changes indetail and arrangement may be made without departing from the spirit andscope of this disclosure.

What is claimed is:
 1. A method for delivering defibrillating energy tothe heart of a patient experiencing abnormal cardiac rhythm, includingventricular fibrillation, which is characterized by an irregular ECGwaveform, and high rate tachycardia and low rate tachycardia, both ofwhich are characterized by regular R-waves occurring at generally stablerates, the deliverance of defibrillating energy taking placeirrespective of the regularity of the ECG waveform representing suchabnormal cardiac rhythm, said method comprising the steps of:monitoringan ECG signal from the heart of the patient; examining said ECG signalto detect said abnormal cardiac rhythm including ventricularfibrillation, high rate tachycardia, and low rate tachycardia; examiningsaid ECG signal to sense heart rate so as to distinguish betweenventricular fibrillation and high rate tachycardia, on the one hand, andlow rate tachycardia, on the other hand, said distinguishing being madewithout regard to the regularity or lack of regularity of the ECGwaveform corresponding to said ECG signal; issuing a defibrillatingsignal when both examining steps sense ventricular fibrillation or highrate tachycardia; not issuing a defibrillating signal when bothexamining steps sense low rate tachycardia or normal sinus rhythm; andresponding to said defibrillating signal by automatically deliveringdefibrillating energy to the heart of the patient.
 2. The method ofclaim 1, wherein said monitoring step comprises sensing anelectrocardiograph (ECG) of the heart to develop ECG data, and saidfirst recited examining step comprises processing said ECG data inaccordance with a probability density function.
 3. The method of claim1, comprising the additional step, prior to said monitoring step, ofconnecting a base electrode and an apical electrode to said heart, saidbase and apical electrodes being used during said monitoring step. 4.The method of claim 1, comprising the additional step, prior to saidmonitoring step, of connecting a base electrode, an apical electrode anda sensing button to said heart, said base and apical electrodes beingused during said monitoring step, and said sensing button being usedduring said monitoring step.
 5. The method of claim 1, wherein said twoexamining steps are performed simultaneously.
 6. The method of claim 5,wherein said monitoring step comprises sensing an electrocardiograph(ECG) of the heart to develop ECG data, and said first recited examiningstep comprises processing said ECG data in accordance with a probabilitydensity function to produce a detection output when said abnormalcardiac rhythm is detected, said method comprising the additional stepof inhibiting said detection output when said sensing step determinesthe presence of low rate tachycardia.
 7. The method of claim 1, whereinsaid first recited examining step and said second recited examining stepare performed in sequence, said second recited examining step beingperformed only when said abnormal cardiac rhythm is detected during saidfirst examining step.
 8. The method of claim 7, comprising theadditional step, prior to said monitoring step, of connecting a baseelectrode, an apical electrode and a sensing button to said heart, saidbase and apical electrodes being used during said detecting step, andsaid sensing button being used during said sensing step.
 9. The methodof claim 1, wherein said second recited examining step comprisescomparing the heart rate of the heart to a predetermined threshold, saidstep of automatically delivering defibrillating energy to the heartbeing executed when said heart rate exceeds said predeterminedthreshold.
 10. The method of claim 9, wherein said second recitedexamining step further comprises automatically returning to said firstrecited examining step when said heart rate remains below saidpredetermined threshold for a predetermined time.
 11. A system fordelivering defibrillating energy to the heart of a patient experiencingabnormal cardiac rhythm including ventricular fibrillation which ischaracterized by an irregular ECG waveform, and high rate tachycardiaand low rate tachycardia, both of which are characterized by regularR-waves occurring at generally stable rates, the deliverance ofdefibrillating energy taking place irrespective of the regularity of theECG waveform representing such abnormal cardiac rhythm, said systemcomprising:monitoring means for deriving an ECG signal from the heart ofthe patient; detecting means, receiving said ECG signal from saidmonitoring means for detecting said abnormal cardiac rhythm, saidabnormal cardiac rhythm including ventricular fibrillation, high ratetachycardia, and low rate tachycardia, said detecting means normallyissuing a defibrillating signal upon detection of said abnormal cardiacrhythm; automatic defibrillating means responsive to said defibrillatingsignal for automatically delivering defibrillating energy to the heartof the patient; sensing means receiving said ECG signal from saidmonitoring means and simultaneously operative with said detecting meansfor sensing heart rate so as to distinguish between ventricularfibrillation and high rate tachycardia, on the one hand, and low ratetachycardia, on the other hand, said distinguishing being made withoutregard to the regularity or lack of regularity of the ECG waveformcorresponding to said ECG signal; and means responsive to said sensingof said ventricular fibrillation and high rate tachycardia by saidsensing means for permitting said defibrillating signal to activate saidautomatic defibrillating means and responsive to said sensing of lowarate tachycardia by said sensing means for preventing deliverance ofsaid defibrillating signal to said automatic defibrillating means. 12.The system of claim 11, wherein said monitoring means comprises asensing circuit for sensing an electrocardiograph (ECG) of the heart todevelop ECG data, and said detecting means comprises processing meansfor processing said ECG data in accordance with a probability densityfunction.
 13. The system of claim 11, further comprising a baseelectrode and an apical electrode connected to said heart, and means forconnecting said base and apical electrodes to said detecting means andsaid sensing means.
 14. The system of claim 11, wherein said monitoringmeans comprises a sensing circuit for sensing an electrocardiograph(ECG) of the heart to develop ECG data, and said sensing meanscomprising a low pass filter circuit connected to said sensing circuitfor low pass filtering said ECG data to provide a low pass filteroutput, said sensing means further comprising a rate circuit connectedto said low pass filter circuit for receiving said low pass filteroutput and responsive thereto for sensing the heart rate.
 15. The systemof claim 11, wherein said detecting means and said sensing means operatesimultaneously, said detecting means including a processing circuit forprocessing said ECG signal in accordance with a probability densityfunction to develop a detection output, said sensing means comprising arate circuit responsive to detection of low rate tachycardia forinhibiting said detection output of said processing circuit, whereby toinhibit automatic defibrillation of the heart of the patient upondetection of said low rate tachycardia.
 16. The system of claim 11,wherein said monitoring means comprises a base electrode, an apicalelectrode and a sensing button connected to said heart, and said systemfurther comprises connecting means for connecting said base and apicalelectrodes to said detecting means, and for connecting said sensingbutton to said sensing means.
 17. The system of claim 16, wherein saidconnecting means comprises a switch circuit having a first state and asecond state, said switch circuit being initially in said first stateduring operation of said detecting means for detecting said abnormalcardiac rhythm, said switch circuit being responsive to detection ofsaid abnormal cardiac rhythm by said detecting means so as to beactuated to said second state, said switch circuit being responsive todetermination of one of said ventricular fibrillation and said high ratetachycardia so as to be actuated to said first state for automaticdefibrillation of the heart of the patient by said automaticdefibrillating means.
 18. The system of claim 17, wherein said detectingmeans comprises a probability density function circuit for determiningwhen said abnormal cardiac rhythm exists, and a bistable circuitconnected to said probability density function circuit and responsive todetermination of the existence of said abnormal cardiac rhythm by saidprobability density function circuit for being actuated to a first stateso as to emit a first output indicating existence of said abnormalcardiac rhythm.
 19. The system of claim 18, further comprising means forproviding said first output of said bistable circuit to said switchcircuit, said switch circuit being responsive thereto for being actuatedto said second state.
 20. The system of claim 19, further comprisingtimed reset circuit means for automatically resetting said bistablecircuit, and means for providing said first output of said bistablecircuit to said timed reset circuit means for actuating said timed resetcircuit means to start counting to a predetermined final count, saidtimed reset circuit means resetting said bistable circuit upon reachingof said predetermined final count.
 21. The system of claim 18, whereinsaid sensing means comprises a heart rate circuit responsive to saidfirst output of said bistable circuit for starting a heart ratemonitoring operation.
 22. The system of claim 21, wherein said heartrate circuit issues a first output when said monitored heart rate of thepatient exceeds a predetermined threshold.
 23. The system of claim 22,further comprising AND gate means responsive to said first output ofsaid bistable circuit and said first output of said heart rate circuitfor actuating said automatic defibrillating means to defibrillate theheart of the patient.
 24. The system of claim 23, wherein said AND gatemeans is responsive to said first output of said bistable circuit andsaid first output of said heart rate circuit for actuating said switchcircuit to said first state.
 25. The system of claim 23, wherein saidAND gate means is responsive to said first output of said bistablecircuit and said first output of said heart rate circuit forautomatically resetting said bistable circuit.
 26. The system of claim18, further comprising timed reset circuit means responsive to issuanceof said first output by said bistable circuit for starting a countingoperation so as to count to a final count value, said timed; resetcircuit means being responsive to reaching of said final count value forresetting said bistable circuit.
 27. The system of claim 17, whereinsaid switch circuit, in said first state, connects said base and apicalelectrodes to said detecting means, and, in said second state, connectssaid sensing button to said sensing means,
 28. The system of claim 17,further comprising interface means for interfacing said switch circuitto said detecting means and said sensing means, said switch circuit, insaid first state, connecting said base and apical electrodes to saidinterface means, and, in said second state, connecting said sensingbutton to said interface means.
 29. The system of claim 28, furthercomprising an ECG amplifier for connecting said interface means, on theone hand, to said detecting means and said sensing means, on the otherhand.
 30. The system of claim 17, further comprising an ECG amplifierfor connecting said switch circuit, on the one hand, to said detectingmeans and said sensing means, on the other hand.
 31. The system of claim11, wherein said monitoring means comprises an ECG input circuit forsensing an ECG of the heart to develop ECG data, and said detectingmeans comprises a processing circuit for processing said ECG data inaccordance with a probability density function, and for issuing a firstoutput when said probability density function is satisfied, saiddetecting means further comprising a bistable circuit responsive to saidfirst output of said processing circuit for issuing a further outputsignal indicating detection of said abnormal cardiac rhythm.
 32. Thesystem of claim 31, wherein said monitoring means comprises a baseelectrode, an apical electrode, and a sensing button connected to saidheart, and said system further comprises connecting means for initiallyconnecting said base and apical electrodes to said ECG input circuit,said connecting means being responsive to said further output signalfrom said bistable circuit for connecting said sensing button to saidsensing means.
 33. The system of claim 31, wherein said sensing meanscomprises a heart rate circuit for monitoring the heart rate of thepatient, said heart rate circuit being responsive to said further outputsignal from said bistable circuit for beginning its heart ratemonitoring operation.
 34. The system of claim 33, wherein said heartrate circuit issues an output signal when said heart rate exceeds apredetermined threshold, said system further comprising AND gate meansresponsive to said further output signal from said bistable circuit andsaid output signal of said heart rate circuit for issuing an outputsignal actuating said automatic defibrillating means to defibrillate theheart of the patient.
 35. The system of claim 34, further comprisingmeans for providing said output-signal of said AND gate means to saidbistable circuit, said bistable circuit being responsive thereto forresetting itself so as to stop issuing said further output signal. 36.The system of claim 34, further comprising a base electrode, an apicalelectrode, and a sensing button, and connecting means for initiallyconnecting said base and apical electrodes to said ECG input circuit,said connecting means being responsive to said further output signal ofsaid bistable circuit for connecting said sensing button to said heartrate circuit.
 37. The system of claim 36, further comprising means forproviding said output signal of said AND gate means to said connectingmeans, said connecting means being responsive to said output signal ofsaid AND gate means for connecting said base and apical cup electrodesto said automatic defibrillating means.